In place of a conventional halogen lamp, a long-life and maintenance-free LED has been popular as a light source used for a vehicle-mounted head lamp. Since the LED has a long life and can ensure required brightness with lower power, and further can keep almost stable brightness by simple control that supplies a constant current thereto, it is suitable for a vehicle-mounted light source.
In addition, since the LED generally exhibits a constant voltage characteristic during lighting, which makes it difficult to control an amount of emission thereof based on an application voltage, it is typical to carry out the lighting through the use of a lighting device of constant-current control that outputs a constant current.
Incidentally, when the LED is under lighting with the lighting device of constant-current control, if the LED or a wiring for connecting the LED is disconnected, the lighting device increases an output voltage thereof in order to keep an output current thereof constant, so that excessive charges are accumulated in a smoothing capacitor of a DC/DC converter. In the case where the disconnection is restored in such a situation, an excessive current due to the charges discharged by the smoothing capacitor is electrified to the LED when the output current starts to flow, which may deteriorate the LED due to the over-current.
It is known that the disconnection and restoration (namely, intermittence) is induced by vibrations of the vehicle, for example, when the connection of a wiring or a connector is incomplete, and for a configuration coping with the intermittent connection, Patent Documents 1 to 4, for example, are proposed.
In Patent Document 1, a lighting circuit for lighting an LED includes a DC/DC inverter whose switching regulator applies to the LED an output voltage based on a source voltage received from a DC power source externally provided to thereby feed a supply current to the LED, and an abnormality of the lighting circuit is detected based on at least one of the output voltage, supply voltage and source voltage of the switching regulator, to thereby stop the switching regulator from outputting. In particular, when the abnormality is detected based on the source voltage, the output voltage of the switching regulator is decreased or stopped, and when the abnormality becomes undetected, the operation of the switching regulator is restarted.
In Patent Document 2, in a lighting control device for lighting an LED, in the event that a chattering phenomenon such that a lead wire connected with the LED is released from a contactor and then connected with the contactor again, in order to avoid a matter that an over-current flown in the LED causes a failure thereof, the over-current is suppressed in such a manner that a resistor is connected in series when the current supplied from a switching regulator to the LED is in a transient state. In addition, the device includes an NMOS transistor for bypassing the resistor connected in series during a normal lighting.
In Patent Document 3, a driving device for lighting an LED includes a voltage clamp unit that is connected in parallel to an output unit of a DC/DC converter, and that suppresses an abnormal voltage when a voltage supplied from the DC/DC converter to the LED varies temporarily due to disconnection or the like.
In Patent Document 4, in a lighting device serving to reduce the brightness of an LED by intermitting a current flown through the LED connected to the output side of a DC/DC converter, an NMOS transistor for intermittence is designed to have a constant-current characteristic in order to suppress an overshoot current produced in an intermittent operation.